Classical Electromechanical Instrument PDF

Classical Electromechanical Instrument  Deflection Instruments Fundamentals They have a pointer deflects over its scale to indicate the quantity to be measured. Three forces are operating inside the instrument. {Deflecting, Controlling, and Damping Forces}  Deflection Instruments...

Classical Electromechanical Instrument  Deflection Instruments Fundamentals They have a pointer deflects over its scale to indicate the quantity to be measured. Three forces are operating inside the instrument. {Deflecting, Controlling, and Damping Forces}  Deflection Instruments Fundamentals Examples ❑Permanent Magnet Moving Coil (PMMC) instruments, Electro- dynamic instruments, Moving iron instruments Deflection Instruments Fundamentals  Deflection Instruments Fundamentals Deflecting force deflects the pointer to a deflecting angle proportional to the input quantity to be measured.  Its direction is towards the full scale deflection angle.  Its magnitude is proportional to the input quantity to be measured. Deflection Instruments Fundamentals  Deflection Instruments Fundamentals Controlling force is generated due to two Spiral control springs in case of Jewel bearing suspension.  Deflection Instruments Fundamentals Where as the taut band control force is generated in case of taut band suspension  Deflection Instruments Fundamentals Controlling force Stops the pointer at its exact - final position. It Returns the pointer to its zero position. Its magnitude is proportional to the angle of deflection Φ  Deflection Instruments Fundamentals The correct damping have a fast and zero oscillation of the pointer movement. Deflection Instruments Fundamentals  Deflection Instruments Fundamentals Magnitude of the damping force is proportional to the pointer acceleration The direction of the eddy current damping force opposes the motion of the coil. Methods Of Supporting The Moving System Of Deflection Instrument  SUSPENSION The pointed ends of pivots fastened to the coil are inserted into cone-shaped cuts in jewel bearings  SUSPENSION Some jewel bearings are spring supported to absorb such shocks more easily  SUSPENSION - The most sensitive jeweled-bearing instruments give full scale deflection (FSD) with a coil current of 25 µA  SUSPENSION Two flat metal ribbons (phosphor bronze or platinum alloy) are held under tension by springs to support the coil  SUSPENSION The ribbons also exert a controlling force as they twist, and they can be used as electrical connections to the moving coil  SUSPENSION With taut-band suspension instruments give FSD with a coil current may be little as 2 µA.  Permanent Magnet Moving Coil (PMMC) Instruments The PMMC Inst. Is The Most Common Used As Deflection Type Instrument.  PMMC Instrument Construction -A permanent magnet with two soft-iron pole shoes - A cylindrical soft- iron core is positioned between the shoes  PMMC Instrument Construction One of the two controlling spiral springs is shown. One end of this spring is fastened to the pivoted coil, and the other end is connected to an adjustable zero- position control.  PMMC Instrument Construction - The current in the coil must flow in one direction to cause the pointer to move from the zero position over the scale.  PMMC Instrument Construction - The terminals (+) and (–) indicate the correct polarity for connection, and the instrument is said to be polarized  PMMC Instrument Construction - It cannot be used directly to measure alternating current Without rectifiers, it is purely a dc instrument Permanent Magnet Moving Coil Instrument  The mirror is placed below the pointer to get the accurate reading by removing the parallax. Torque Equation & Scale  Torque Equation & Scale The force F affecting on both sides of the Coil ( N turns) ┴ to B. F = BILN They produce a deflecting torque Tdef Tdef = BILND Tdef = BINA = Cdef I  Torque Equation & Scale Where A is the area of one turn of the coil [m2], Cdef = BNA is the deflection constant …......[Nm/Ampere]  Torque Equation & Scale As Tcon α Φ Tcon = Ccon Φ Where Ccon is the control constant [Nm/degree]. At final position of the pointer: (Tdef) = (Tcon) Then Φ = KI Where K = Cdef / Ccon  Torque Equation & Scale Conclusion: The pointer deflection is linearly proportional to I. The PMMC scale is linear (equally spaced). If the current changes its direction(-ve current), the pointer will deflect off the scale.  PMMC Instrument Is Called A Polarized Instrument Its deflection depends on the polarity of its input quantity. It cannot be used to measure an (ac) directly, but a rectifier must be used firstly to convert (ac) quantity to (dc) quantity before applying it to instrument.  Advantages Of The PMMC Instruments Linear scale. Simple and cheap. Can be constructed with very high sensitivity (specially if taut band suspension is used).  Disadvantages Of The PMMC Instruments Polarized External magnetic fields badly affect its operation.This can be avoided by using core magnet type PMMC construction****. Not very sensitive (to have sensitive device the taut band suspension must be used: which is expensive).  EXAMPLE: A PMMC inst. with a 100-turn coil has a magnetic flux density in its air gaps of B = 0.2T. The coil dimensions are D=1 cm and L=1.5 cm. calculate the torque Tdef on the coil for a current of 1mA and its deflection constant (Cdef). If the device constant K = 12x103 degree/A, find: The spring (control) constant Ccon. Find the angle of deflection (Φ) for the input currents: 1,2,4, and 8mA. Conclude your results.  SOLUTION: Tdef =BILND [Nm] Tdef=0.2x1x10-3 x1.5x10-2mx100x1x10-2m =3x10-6 Nm. Tdef = Cdef I Cdef = Tdef / I = 3x10-6 / 1x10-3 Nm/ A K= Cdef / Ccon Ccon= (3x10-3 Nm/A)/ (12x103 0/A) Nm/degree. Φ=KI =12x103x1x10-3=120 = 12x103x2x10-3=240 = 12x103x4x10-3=480 =12x103x4x10-3=960  Galvanometer Instrument A Galvanometer is a PMMC instrument designed to be : Very sensitive to very low currents Null detectors and its zero deflection is in mid-scale.  Galvanometer Instrument  In order to achieve these requirements,  Taut band suspension is used to increase its sensitivity.  light beam pointer is used.  using shunt damping resistor which controls the level of eddy currents.  Galvanometer Instrument  Critical damping resistance value which gives just sufficient damping to allow the pointer to settle down quickly with a very small short lived oscillation. Rcd = SV / SI [Ω, μV /mm / μA /mm]  Galvanometer Instrument Current Sensitivity (SI) [μA /mm] It is amount of current (μA ) flowing through the instrument to give deflection (mm)..  Galvanometer Instrument Voltage Sensitivity (SV) [μV /mm] It is expressed for a given value of a damping resistance as follows: SV = Rcd SI [μV /mm, Ω μA /mm].  Galvanometer Instrument Megohm sensitivity (SMΩ) [MΩ] It is the resistance connected in series with the instrument to restrict the deflection to one scale division for 1 V potential difference between its terminals SV = Rcd SI [μV /mm, Ω μA /mm]  Protection Of The Galvanometer When the galvanometer is used as a null detector (e.g. across the diagonal of a Wheatstone bridge), initially high current may passes across it due to the unbalance of the bridge.  Protection Of The Galvanometer : EXAMPLE A galvanometer (a) has a current sensitivity of 1 μA/mm and a critical damping resistance of 1KΩ. Calculate its voltage sensitivity.  Another galvanometer (b) deflects by 2 cm when its current is 10 μA, find its current sensitivity, and critical damping resistance if its voltage sensitivity is 2mV/mm. SOLUTION (a) SV = Rcd SI = 1KΩ x 1 μA/mm = 1 mV /mm (b) Si = current (μA) / deflection in mm = 10 μA / 20 mm = 0.5 μA/mm Rcd = SV / SI = 2 mV/mm / 0.5 μA/mm = 4 KΩ